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Thoughts on Nylas’ “How We Deploy Code”

The goals of making deployment consistent, reliable and fast are very laudable, and the conclusion involving creating Debian packages is just great. But in the spirit of geek nitpicking, I can’t help but think the justifications given are misguided, and overlook a simpler solution.

>> pip does not offer a “revert deploy” strategy

Yes it does. Throw away the virtualenv and create a new one using the requirements.txt from an earlier release. This might be slow, but you can both speed it up (see below), and you can keep old versioned virtualenvs around, as a cache of the output of this slow step, so that reverts (the time when you really want deploys to go quickly) require only a couple of symlinks.

Update: Oooh, and I just had an idea. You could version virtualenvs using a hash of the requirements, so that deploys which do not change dependencies can share the same virtualenv. I’ve never tried that – it might work?!?!

>> Installing dependencies with pip can make deploys painfully slow

This is true. But it’s not the final word on the matter.

Firstly, don’t grab the dependencies you’re installing from PyPI. Have a local cache of them. That speeds up the install tremendously, not just because no download takes place, but also because no trawling of PyPI and author sites for possible alternate versions takes place. Some people use a local PyPI instance, but I like simply using a directory full of packages. Point pip at it using ‘pip install --no-index --find-links=packages -r requirements‘. The ‘packages’ directory could be checked into your project’s repo, so that once you’ve checked a project out, you have everything you need to deploy to local VMs, even with no network connection at all. I wrote about this a while ago.

>> Building your code separately on each host will cause consistency issues

So don’t install dependencies using source packages, use wheels instead. Then any slow or unreliable build step is done once, when you create the packages directory, while deployment is now reliable and requires no development tools on the production server such as compilers, headers, etc.

Update: As a bonus, this will again substantially speed up the creation of your virtualenv when deploying.

>> Deploys will fail if the PyPI or your git server are down

PyPI outages (or a package author deleting their package, which happens routinely) are not a problem if you’re deploying from a local source of packages.

I agree that application code deployment shouldn’t be done using a ‘git pull’. Accessibility to GitHub shouldn’t be a single point of failure, and the fewer development tools needed on production servers the better. So export the code from git into a tar file when you cut a release, and then push it out using scp at deploy time.

Having said all that, there are still advantages to having your whole app and its dependencies handled by a single mechanism like Debian packages, rather than more bug-prone ad-hoc scripts and Ansible config. So I’m not at all against the final conclusions of the Nylas article. (Hugs to you folks at Nylas!)

Dashed this out in ten minutes between daycare pickup and dinner being ready. Apologies for the inevitable shoddyness.